The disclosed invention is directed generally to hybrid multilayer circuit structures, and is directed more particularly to layered planar transmission lines formed in a hybrid multilayer circuit structure.
Hybrid multilayer circuit structures, also known as hybrid microcircuits, implement the interconnection and packaging of circuit devices (e.g., integrated circuits), and generally include a unitized multilayer circuit structure formed from a plurality of integrally fused insulating layers (e.g., ceramic layers) having conductor traces disposed therebetween. The circuit devices are commonly mounted on the top insulating layer so as not to be covered by another insulating layer or on an insulating layer having die cutouts formed thereon to provide cavities for the circuit devices. Passive components such as capacitors and resistors can be formed on the same layer that supports the circuit devices, for example, by thick film processes, or they can be formed between the insulating layers, for example, also by thick film processes. Electrical interconnection of the conductors and components on the different layers is achieved with vias or holes appropriately located and formed in the insulating layers and filled with conductive via fill material, whereby the conductive material is in contact with predetermined conductive traces between the layers that extend over or under the vias.
Hybrid multilayer circuit structures are now being utilized for RF applications wherein microwave integrated circuits (MICs) and/or monolithic microwave integrated circuits (MMICs) mounted on a hybrid are interconnected by transmission structures formed on the multilayer structure, for example pursuant to screen printing or metal deposition.
A consideration with implementing transmission structures in hybrid multilayer circuit structures is high frequency attenuation due to conductor losses. For example, for a microstrip transmission structure that includes metal on both sides of a substrate, conductor loss is determined by strip width which is constrained by the need for a particular characteristic impedance, which prevents conductor loss from being independently tuned. Similarly, for a transmission structure comprised of a stripline that is buried between two wide conductive plates, conductor loss is determined by the stripline width which is constrained by the need for a particular characteristic impedance.
For a coplanar waveguide comprised of coplanar conductive lines, strip width and thickness can be selected to reduce conductor loss, and then gap width is constrained by the desired characteristic impedance. However, increasing strip width might not appreciably reduce losses since the current crowds the edges of the metallization, and increased strip thickness might constrained by process limitations.